FR2968092A1 - DOPED OPTICAL FIBER IN RARE EARTHS THAT IS INSENSITIVE TO IRRADIATION - Google Patents

Abstract

The invention relates to an optical fiber comprising, from the center towards the periphery: - a central core (11) suitable for transmitting and amplifying an optical signal, the central core (11) being made up of a core matrix comprising nanoparticles, the nanoparticles being formed of a nanoparticle matrix comprising doping elements from the group of rare earths; - an optical sheath surrounding the central core (11) adapted to confine the optical signal transmitted by the central core (11), the optical sheath having a plurality of holes (10) which extend along the length of the optical fiber, the holes (10) being separated from each other by a pitch (Λ); -an outer sheath.

Description

The present invention relates to the field of optical fibers, and more specifically, relates to a rare earth doped optical fiber insensitive to irradiation. The invention also relates to a method of manufacturing such an optical fiber. An optical fiber conventionally comprises a central core, whose function is to transmit and possibly to amplify an optical signal, and an optical cladding, whose function is to confine the optical signal in the central core. To ensure this function of guiding the optical signal, the refractive index of the central core nc is greater than the refractive index of the sheath ng (Le nc> ng). Typically, the difference in refractive index between the central core and the sheath is obtained using doping elements inserted in the central core and / or the sheath. Optical sheath is understood as opposed to the outer sheath constituted by the refilling of the primary preform of the optical fiber. Typically, the central core and the optical cladding are obtained by a gas phase deposition, such as a MCVD deposit ("Modified Chemical Vapor Deposition" in English), OVD ("Outside Vapor Deposition" in English), VAD ("Vapor Axial Deposition "in English) or others. In the case of a MCVD type process, the outer sheath consists of the deposition tube and possibly a refill or sleeving. In general, low volatility elements such as aluminum or rare earth elements (hereinafter referred to as "rare earths") are incorporated by impregnating a porous silica (SiO 2) layer obtained for example during an intermediate step of the MCVD process. In the case of rare earths, the impregnation is for example carried out with a solution comprising rare earths obtained from dissolved salts. In systems using amplification of the optical signal, rare earth-doped optical fibers are commonly used. For example, erbium doped optical fibers are used in EDFAs ("Erbium Doped Fiber Amplifier" in English) for an amplification of the optical signal transmitted in some long distance optical telecommunication systems. EDFAs have high performance in terms of power consumption, and R: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11:11 - 1/33 optical power conversion efficiency . Typically, the optical fibers doped with rare earths have a central core composed of a silica matrix comprising rare earths, such as erbium, at concentrations of the order of 250 to 1000 mass ppm (0.025% to 0%). , 1% by weight or wt%).

The gain form of an amplifying optical fiber designates the gain value as a function of the wavelength of the incident signal. For example, an erbium doped optical fiber is advantageously used in the C band (1530-1565 nm). An erbium-doped optical fiber conventionally has a gain width of about 30-35 nm in the C-band and a numerical aperture of 0.23.

For wavelength division multiplexing (WDM) applications, a large gain width is sought. For this, rare earths are optionally associated with complementary doping elements to improve amplification. Complementary doping elements enhance amplification by preventing rare earth interactions with each other. For this, it is appropriate that the complementary doping elements surround the rare earths. When doping the optical fiber with a solution containing rare earths, the concentration of complementary doping elements is very high, so that each rare earth in the central core can be surrounded by complementary doping elements. Aluminum (Al) is an example of a complementary doping element. Moreover, in applications in environment subject to irradiation, as for example in nuclear power plants, optical systems have several advantages over electronic systems. Indeed, optical fibers have better electromagnetic immunity and better chemical stability. Optical fibers provide more reliable and secure communication systems that require little maintenance. Optical fibers also provide a high data rate capability. Optical fibers also have a high compactness, which makes them particularly suitable for use in embedded systems inside aerospace apparatus. However, environments subject to irradiation cause an increase in the optical fiber background losses, and thus an attenuation of the transmitted signal.

8: 131900/31905 AOB / 31905--101125-texte dépôt.doc - 25/11/10 - 11:11 - 2/33 This increase in bottom losses may be due to defects in the silica structure created by the 'irradiation. Such defects are, for example, pendant links, or trapped charges, which absorb the optical signal transmitted in the optical fiber at certain wavelengths.

The increase in background losses may also be due to the creation of specific defects related to the dopants necessary to provide optical signal amplification and guidance properties. The importance of these dopants in the sensitivity of the optical fiber to irradiation will be better understood with reference to FIG.

Figure 1 shows attenuation values of the transmitted signal in several examples of optical fibers. The curve has on the ordinate radiation-induced attenuation or RIA (Radiation Induced Attenuation) expressed in dB / km / Gy, and abscissa the wavelength of the transmitted signal, expressed in nm. Curve 1 is acquired on an optical fiber having a central core of pure silica, that is to say having no doping elements in the core, and irradiated at 100 Gy. Curve 2 is acquired on an optical fiber exhibiting an aluminum doping, with a concentration of about 7% by weight of aluminum in the central core, and irradiated under 360 Gy. The curve 3 is acquired on an optical fiber having a phosphorus doping, with a concentration of about 10% by weight of phosphorus in the central core, and irradiated at 500 Gy. Curve 4 is acquired on an optical fiber exhibiting germanium doping, with a concentration of approximately 5% by weight of germanium in the central core, and irradiated under 100 Gy. It is observed that the attenuation induced under irradiation depends on the wavelength of the transmitted signal and the radiation-sensitive dopants present in the central core of the optical fiber. Thus, increasing the attenuation of the optical signal in an EDFA depends on the dopants used to obtain predetermined guiding properties, such as germanium, aluminum, phosphorus or fluorine. The increase in signal attenuation also depends on the elements introduced because of the manufacturing process. For example, chlorine is used to prevent contamination of materials by OH- hydroxide ions. The increase in attenuation also depends on the complementary doping elements used to obtain the required amplification properties. These doping elements

For example, they may be aluminum, phosphorus, antimony, lanthanum or bismuth. To reduce the sensitivity of an optical fiber to irradiation, the REGNIER et al. entitled "Recent developments in optical fibers and how defense, security and sensing can benefit", Proceedings of the SPIE, vol. 7306, 2009, pp. 730618-730618-10 proposes to eliminate the complementary doping elements required for the amplification properties, by using a rare earth doping with silica nanoparticles. The nanoparticles consist of a matrix of silica and rare earths. The chemical composition of nanoparticles ensures that rare earths do not form aggregates. As the matrix does not comprise complementary doping elements, the sensitivity to irradiation is therefore reduced. However, the complementary doping elements improve the quality of the amplification. The suppression of complementary doping elements does not allow applications with a large spectral bandwidth, such as WDM. In addition, the silica allows a limited solubility of the rare earths, which limits the gain of amplification. Thus, for doping with erbium, the doping with silica nanoparticles is limited to optical fibers having an amount of erbium of less than 300 ppm by mass. It is also proposed in the prior art to reduce the length of the optical fiber, to reduce the volume of optical fiber exposed to irradiation. Such a reduction is in particular obtained by improving the amplification gain per unit length of the optical fiber. The document by MA et al. entitled "Experimental investigation of radiation effect on erbium-ytterbium co-doped fiber optics for low-dose optical radiation communication environment", Optics Express, vol. 17, No. 18, 2009, pages 15571 to 15577 proposes to increase the amplification efficiency using doping with an erbium / ytterbium combination. But this doping increases the sensitivity of the optical fiber to irradiation. This is partly due to the phosphorus necessary for the transfer of energy from ytterbium to erbium, which is present in the composition of the matrix of the central core. This solution of the prior art is also more expensive than conventional doping with erbium, since it generally comprises double-jacketed structures suitable for high power applications. The document by GUSAROV et al. entitled "Radiation sensitivity of EDFAs based on highly doped fibers", Journal of Lightwave Technology, vol. 27, No. 11,

A: 131900A31905 AOB \ 31905--1011254exposed.doc - 25/11/10 - 11:11 - 4/33 2009, pp 1540 to 1545, proposes, for its part, to increase the amplification efficiency by increasing the amount of rare earths present in the optical fiber. However, when the concentration of rare earths in the central core of the optical fiber is large, we observe the formation of pairs or even rare earth aggregates in the matrix of the central core, which creates doping inhomogeneities. These doping inhomogeneities adversely affect the amplification efficiency of the optical fiber due to the simultaneous existence of non-radiative mechanisms between the rare earths, such as quenching phenomena (or "quenching" in English). These rare-earth cooperative processes are, for example, Homogeneous Up-Conversion (HUC) and Pair-Induced Quenching (PIQ). These mechanisms parasitize the stimulated radiative emission carrying out the amplification. For example, in an optical fiber having a central silica core comprising an aluminum concentration of about 7% by weight, the "quenching" phenomenon becomes important for an erbium concentration of 700 ppm by mass. These untimely energy transfers compete with the emission stimulated by the pump beam, thus limiting the amplification efficiency of the optical fiber. Such rare earth aggregates can also accentuate photonic degradations, such as photon dyeing (or "photodarkening" in English).

These photonic degradations may take place in the central core of the high-power optical fiber during the propagation of light signals, due to highly absorbing defects present in the matrix of the central core. To overcome these aggregates, the quantity of complementary doping elements is increased. The reduction in the volume of optical fiber exposed to irradiation can thus be rendered ineffective by this increase in the amount of radiation-sensitive elements. Erbium-doped amplifying optical fibers are known in which a hole-type cladding makes it possible to reduce the length of the optical fiber while maintaining a high gain of amplification. Holes with holes are known for example from KNIGHT et al. entitled "All silica single-mode optical fiber with photonic crystal cladding", Optics Letters, vol. 21, 1996, pp. Optical fibers comprising a hole-type sheath are referred to as photonic crystal optical fibers or microstructured optical hole or fiber optic fibers or

R: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11:11 - 5/33 still PCFs (for "Photonic Crystal Fibers" in English). For example, perforated optical fibers formed from multilayer film called bragg fiber are known. There are also known optical fibers with holes that confine the signal by using the forbidden energy band.

PCFs provide internal reflection guidance through an array of air holes in the optic fiber optic cladding. The central core of the optical fiber can then consist of pure silica. By pure silica is meant silica which does not include doping elements. The sheath is pure silica with air holes. It is no longer necessary to use dopants to obtain guiding properties. For example, the document by CUCINOTTA et al. entitled 'Design of erbiumdoped triangular photonic-crystal fiber based amplifiers', IEEE Photonics Technology Letters, vol. 16, No. 9, 2004, pp. 2027-2029, and K. FURUSAWA et al. entitled "High gain efficiency based on an erbium doped alumino-silicate holey fiber", Optics Express, Vol 12, No. 15, 2004, pp. 3452 to 3458, propose an optical fiber doped with erbium and comprising a perforated sheath. The perforated sheath makes it possible to improve the amplification properties of these amplifying optical fibers. This is mainly due to the theoretical improvement of the recovery between the rare earth doped section on the one hand and the pump and signal beams on the other hand. However, the documents of CUCINOTTA and FURUSAWA are not interested in the sensitivity of optical fibers to irradiation. The optical fiber comprising a perforated sheath of the document by HILAIRE et al. entitled "Numerical study of single mode Er-doped microstructured fibers: influence of geometrical parameters on amplifying performance", Optics Express, vol. 14, 2006, pp. 10865 to 10877, in which the reduction of the optical fiber length with respect to a conventional erbium doped optical fiber reaches 40%, while maintaining the same gain curve. The profile of the optical fiber is optimized to obtain a 90% overlap, which varies slightly with the wavelength. However, the document does not focus on the sensitivity of the optical fiber to irradiation. The document by S. GIRARD et al. entitled "gamma-radiation induced attenuation in PCF", Electronic Letters, vol. 38, No. 20, 2002, pp. 1169-1171, indicates that the sensitivity of PCFs to irradiation is highly dependent on the purity of

R: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11:11 - 6/33 the silica used to obtain the heart. It is also known, for example, from the document by S. GIRARD et al. entitled "Radiation-induced defects in fluorine-doped silicabased optical fibers: Influence of a pre-loading with Hz", Journal of Non-Crystalline Solids, 355, 2009, pp. 1089 to 1091, that the chlorine in the silica induces a strong absorption under irradiation especially at low wavelengths. However, the documents do not deal with rare earth-doped amplifying optical fibers. In particular, the documents do not deal with the problem of the spectral bandwidth of an amplifying optical fiber. There is therefore a need for a rare earth doped optical fiber which has low irradiation sensitivity and non-degraded amplification properties. For this purpose, the invention proposes an optical fiber comprising, from the center to the periphery: a central core adapted to transmit and amplify an optical signal, the central core consisting of a core matrix comprising nanoparticles, the nanoparticles being formed of a nanoparticle matrix comprising doping elements of the rare earth group; an optical cladding surrounding the central core adapted to confine the optical signal transmitted by the central core, the optical cladding having a plurality of holes which extend along the length of the optical fiber, the holes being separated from one another by not ; - an outer sheath. According to one embodiment, the optical cladding has a chlorine concentration of less than 500 mass ppm and is devoid of other chemical elements having a concentration greater than one part per billion mass. According to one embodiment, the optical cladding has a chlorine concentration of less than 100 mass ppm and is devoid of other chemical elements having a concentration greater than one part per billion mass. According to one embodiment, the optical cladding is made of pure silica. According to one embodiment, the holes and the central core only have rotational symmetries of order 7r modulo n around the center of the optical fiber, where n is an integer. According to one embodiment, the pitch is between 2 microns and 10 microns.

According to one embodiment, the holes have a substantially circular cross section, and each hole has a diameter such as: that the ratio between the diameter and the pitch is between 0.3 and 0.9. According to one embodiment, the core matrix is pure silica.

According to one embodiment, the central core has a rare earth element dopant concentration of between 200 and 1000 mass ppm, and a nanoparticle matrix concentration of between 0.5 and 5% by weight. According to one embodiment, the nanoparticles have an atomic ratio between the nanoparticle matrix and the rare earth element doping elements between 10 and 500, preferably between 50 and 350. According to one embodiment, the nanoparticle matrix is a material selected from alumina, silica, or a combination thereof. According to one embodiment, the doping elements of the rare earth group are chosen from erbium, ytterbium, thulium or a combination thereof. The invention also relates to an optical amplifier comprising at least one portion of optical fiber according to the invention and using a pump power of between 150 mW and 1.5 W. According to one embodiment, the optical amplifier has a width of Band between 25 and 32 nm for the C band (from 1530 to 1560 nm). The invention also relates to an optical fiber optical laser comprising at least one portion of optical fiber according to the invention. The invention also relates to a method for manufacturing a primary optical fiber preform, comprising the steps of: forming a part of the preform comprising nanoparticles doped with elements of the rare earth group, said part of the preform helping to form the central core of the optical fiber; - Making a plurality of capillaries - Arranging the capillaries in a bundle of capillaries comprising said portion 30 of the preform at its center. According to one embodiment, said part of the preform contributing to forming the central core of the optical fiber is made of a pure silica-based material.

According to one embodiment, the capillaries have a chlorine concentration of less than 500 ppm by weight and are devoid of water. other chemical elements with a concentration greater than one part per billion mass. In one embodiment, the capillaries have a chlorine concentration of less than 100 mass ppm and are free of other chemical elements having a concentration greater than one part per billion mass. According to one embodiment, the capillaries are made of pure silica. According to one embodiment, the forming step comprises the steps of: - performing a porous deposit on the inner surface of a tube, said deposit having a tubular shape; impregnating said porous deposit with a suspension of nanoparticles doped with elements of the rare earth group; - vitrify said porous deposit, - shrink the assembly consisting of the tube and the porous deposit; extracting said porous deposit, said deposit constituting said part of the preform contributing to forming the central core of the optical fiber. According to one embodiment, the deposition step is carried out via a modified chemical vapor deposition technique MCVD ("Modified Chemical Vapor Deposition"). According to one embodiment, the forming step comprises the following steps: forming a bar comprising nanoparticles doped with elements of the rare earth group; - vitrify the bar; - Tighten the bar, said bar constituting said portion of the preform contributing to form the central core of the optical fiber. According to one embodiment, the bar is made by means of a sol-gel process. According to one embodiment, the step of manufacturing the plurality of capillaries comprises the steps of: - depositing on the inner surface of a tube, said deposit having a tubular shape;

R: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11:11 - 9/33 - extracting the tubular deposit; - draw capillaries from the tubular deposit. According to one embodiment, the step of manufacturing the plurality of capillaries comprises the steps of: - depositing on the inner surface of a tube, said deposit having a tubular shape; and pulling capillaries from the tube comprising the deposit on its inner surface. According to one embodiment, the step of depositing is carried out by means of a PCVD ("Plasma Chemical Vapor Deposition") physicochemical deposition technique. According to one embodiment, the concentration of nanoparticles in the part of the preform is between 1016 and 1018 / cm3. According to one embodiment, the nanoparticles have a substantially spherical shape and a diameter of between 5 and 25 nm. In an optical fiber according to the invention, the rare earth doping is carried out with nanoparticles comprising rare earths. Doping in the form of nanoparticles makes it possible to limit or even eliminate the use of complementary doping elements, which are sensitive to irradiation. The optical fiber according to the invention also comprises an optical cladding having holes, also called hole sheath. The holes of the optical cladding make it possible to ensure the guiding properties of the optical fiber, without resorting to radiation-sensitive dopants. In addition, the holes make it possible to reduce the length of optical fiber, by increasing the conversion efficiency. Thus, the length of optical fiber exposed to irradiation is reduced. The optical fiber according to the invention is therefore highly insensitive to irradiation. The characteristics of the optical fiber make it possible to obtain irradiation resistance and optimized amplification properties. Other features and advantages of the invention will appear on reading the following description of particular embodiments of the invention, given by way of nonlimiting examples and with reference to the appended drawings, in which: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11:11 - 10/33 - FIG. 1, already described, shows the attenuation induced under irradiations in an optical fiber as a function of the dopants present. in the optical fiber, and the wavelength of the signal transmitted by the optical fiber; FIG. 2 shows an example of an optical fiber according to the invention; - Figure 3 shows another example of optical fiber according to the invention; FIG. 4 shows yet another example of optical fiber according to the invention; FIG. 5 is a diagram illustrating steps of a method according to the invention; FIG. 6 is a diagram illustrating steps of a method according to the invention, in a particular embodiment; - Figure 7 is a diagram illustrating steps of a method according to the invention, in a particular embodiment; and FIG. 8 shows a sectional view of an optical fiber according to the invention in a particular embodiment.

The optical fiber according to the invention will be described with reference to FIGS. 2, 3, 4 and 8 which show sectional views of examples of optical fibers according to the invention. The figures show the optical fibers in a plane perpendicular to their axis. The optical cladding is made of a material adapted to the transmission of an optical signal, such as silica. The optical cladding includes holes 10 that extend along the length of the optical fiber. In other words, the holes 10 extend in a direction parallel to the axis of the optical fiber. The holes 10 form a regular network of holes in which the axes of two adjacent holes are spaced afro.

In a transverse plane, perpendicular to the axis of the optical fiber, the sections of the holes 10 are organized in the form of a periodic triangular pitch grating. The periodic grating is obtained from a pattern of three holes forming an equilateral triangle, with the exception of a central zone occupied by the central core 11 of the optical fiber.

In addition, the holes 10 are arranged in concentric hole rings. The term "crown" is understood here in the broadest sense, including in particular a substantially hexagonal shape as illustrated in FIGS. 2, 3, 4 and 8.

The examples presented in FIGS. 2, 3, 4 and 8 are not limiting. In particular, the holes 10 may be arranged according to another periodic network. The concentric crowns may take a geometric shape other than a hexagonal shape.

Holes 10 are filled with a gas, such as air or carbon dioxide (CO2). The medium inside the holes 10 is different from the material between the holes 10. Thus, the medium inside the holes 10 has a different refractive index of the material around the holes 10. The number and the volume of the holes 10 make it possible to adjust the refractive index of the optical cladding. Thus, the number and the volume of the holes 10 make it possible to adjust the numerical aperture of the optical fiber and the index difference of the central core 11 of the optical fiber with respect to the optical cladding. Thus the holes 10 of the optical cladding make it possible to obtain a difference in index between the central core 11 and the optical cladding without using doping elements for this purpose. The confinement of the optical signal in the central core 11 is obtained without the presence of doping elements. The quantity of dopants sensitive to irradiation is therefore reduced. For example, in the particular embodiment illustrated in FIG. 8, the holes 10 have a circular cross-section and a diameter (D.sub.rho..times..times..times..times..times..times..times..times..times..times..times..times..times..times..times..times..times..times..times..times..times..times..times .. 0.3 to 0.9 The optical cladding has between 2 and 6 concentric rings of hexagonal shaped holes The holes 10 may vary from one another due to the uncertainties of the optical fiber manufacturing process. Holes 10 have a tolerance of 20% on pitch and diameter values (D> rou.

In other embodiments, the holes 10 have a circular cross-section and different diameters (I)> rot. In the particular example presented in FIG. 2, holes whose centers are situated on a straight line passing through the center of the optical fiber have a diameter smaller than that of the other holes of the optical cladding. In another particular example shown in FIG. 3, diametrically opposed holes belonging to the first ring have a diameter smaller than that of the other holes. The holes 10 of the optical cladding also make it possible to improve the overlap between the rare earth doped zone 12 and the pump optical beam.

8: 131900/31905 AOB / 31905--101125-text filing.doc - 25/11/10 - 11:11 - 12/33 and / or the overlap between the rare earth doped area 12 and the signal optical beam. This improvement in recoveries between the zone 12 doped with rare earths and the pump, and / or the signal, makes it possible to optimize the length of the optical fiber. For the same concentration of rare earths and for pump and signal beams respectively having a wavelength of 980 nm and 1550 nm, the optical fiber according to the invention has a decrease in the length of optical fiber between 20 and 50%, with respect to an optical fiber of the prior art in which the index difference of the central core 11 with the optical cladding is obtained with doping elements. Thus, the optical fiber according to the invention has an optical fiber length exposed to irradiations which is less than the length of optical fiber exposed in an optical fiber of the prior art. The number and the volume of the holes 10 further determine other optical properties of the optical fiber. The pitch At. Has a value which ensures a compromise between the effective area (in English "effective area") of the optical fiber, and the curvature losses. In the example, where the holes 10 have a circular cross section and a diameter (D> rou, the pitch Afro 'and the ratio (Dtrou / Afro' also ensure a stability of the monomode behavior of the optical fiber for the lengths d the waves used in the optical fiber, for example in an amplification operation Thus, the pitch Afro 'and the ratio (Dtrou / Afro' ensure that the cutoff wavelength of the optical fiber Xc is less than the length The optical cladding has been described with holes 10 of circular cross-section, but the described example is not limiting. may have a cross section of a shape other than circular.

The central core 11 is the portion of optical fiber located at the center of the optical fiber and surrounded by the optical cladding. In a plane perpendicular to the axis of the optical fiber, the central core 11 is the zone lying inside a curve tangential to the first ring of holes 10 from the center of the optical fiber. In one embodiment, the central core 11 occupies a single point of the periodic point network described above, located in the center of the optical fiber. In an example according to this embodiment, shown in FIG. 8, where the holes 10 have a circular cross-section and a diameter (D> rou), the central core 11 has a circular section having a radius ri equal to 2XAtro '- (Dtrou .

R: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11:11 - 13/33 In other examples according to this embodiment shown in Figures 2 and 3, in which the holes 10 of the first ring have different diameters, the core 11 has a cross section of elliptical shape. In another embodiment, the central core 11 comprises the point of the network located in the center of the optical fiber and one or more other points of the network. Figure 4 shows an example of this embodiment. The central core 11 has a cross section having substantially the shape of a quadrilateral. The central core 11 comprises a core matrix and nanoparticles. The core matrix surrounds the nanoparticles doped with rare earths. The core matrix contributes to guiding the optical signal in the central core 11. The core matrix is devoid of doping elements. In particular, the core matrix is devoid of doping elements to obtain a difference in index between the central core 11 and the optical cladding. Thus, the core matrix does not contain doping elements causing attenuation of the signal under irradiation. The core matrix 11 has a chlorine concentration of less than 1000 mass ppm. Typically, chlorine is used (as for example in a material such as pure silica), to avoid contamination with OH- hydroxide ions. However, chlorine leads to high absorption under irradiation in the range of visible wavelengths. By minimizing the amount of chlorine in the optical fiber, the irradiation resistance of the optical fiber is improved. The core matrix 11 is further devoid of other detectable chemical elements by conventional chemical analysis means. The plasma torch or ICP (Inductively Coupled Plasma) is an example of a chemical analysis. For example, the core matrix 11 is devoid of other chemical elements having a concentration greater than one part per billion mass or ppb ("part per billion" in English). In other words, in the core matrix, the concentration of chemical elements other than chlorine is less than one part per billion mass. Thus, the matrix of the central core 11 has a limited amount of chemical elements, which would attenuate the signal transmitted by the optical fiber. The core matrix has attenuation of the signal under irradiation which is minimized. The

R: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11:11 - 14/33 heart matrix is devoid of chemical elements inducing losses under irradiation greater than 0.05 dB / m / kGy in a range of useful wavelength. By range of useful wavelength is meant a range of wavelength in which is included the wavelength of the signal transmitted by the fiber. For example for erbium, the useful wavelength range is 900 to 1600 nm. The core matrix is for example a vitreous matrix mainly composed of silica. The core matrix 11 is for example pure silica. By pure silica is meant silica having no doping elements. The central core 11 further comprises nanoparticles. The nanoparticles are surrounded by the heart matrix. The doping of the central core 11 with rare earths is obtained via the nanoparticles. The nanoparticles are comprised in a central zone 12 of the central core 11, of substantially circular cross section with a doping diameter. The doping diameter of the doped zone 12 is between 2 and 10 μm. The doping diameter has a maximum value making it possible to preserve the constraints on the ratio (Dtrou / Afro ,,, especially during the manufacture of the optical fiber.) The nanoparticles are formed of a nanoparticle matrix comprising rare earths. nanoparticle surrounds rare earths The composition and structure of the nanoparticle matrix promote the solubilization of rare earths.This nanoparticle matrix is independent of the composition of the core matrix 11 of the optical fiber.For example, the nanoparticle matrix is in silica, aluminum or a combination of them.Rare earths allow the amplification of the signal transmitted by the central core 11. In the optical fiber, the rare earths are in the ionized form of the same element The rare earth is, for example, erbium (Er), ytterbium (Yb), thulium (Tm), or a combination thereof, or all a rare earth allowing amplification by optical pumping. For example, the rare earth is erbium for amplification in the C band.

Doping in the form of nanoparticles makes it possible to avoid the aggregation of the rare earths between them. It is therefore not necessary to saturate the central core 11 with complementary doping elements to avoid an aggregation of the rare earths between

R: 131900A31905 AOB \ 31905--101125-text deposit.doc - 25/11/10 - 11:11 - 15/33 them. Thus, the amplification properties are preserved, while reducing the amount of radiation-sensitive elements in the central core 11. For example, the doped zone 12 of the central core 11 has a rare earth concentration of between 200 and 1000 ppm mass and a concentration of nanoparticle matrix of between 0.5 and 5% by weight, preferably between 1.5 and 4% by weight. For example, the nanoparticles have an atomic ratio between the nanoparticle matrix and the rare earths which is between 10 and 500, advantageously between 50 and 350, or even between 50 and 200.

The fiber also has a typical outer sheath of an optical fiber. For example, the outer sheath is made of natural silica for cost reasons. In another example, the outer optical cladding is doped silica. By virtue of its characteristics, the optical fiber according to the invention is from 2 to 100 times, preferably from 10 to 1000 times, depending on the irradiation and operating conditions of the optical fiber, which is more resistant to irradiation than an optical fiber. of the prior art, with equal amplification performance. Thus, the optical fiber according to the invention has an attenuation induced under irradiations of between 0.005 dB / m / kGy and 0.05 dB / m / kGy, in the range of useful wavelength. In addition, thanks to the small amount of complementary dopants used, or even the absence of complementary dopants, the optical fiber according to the invention has low background losses before irradiation. Thus, the background losses before irradiation are less than 5 dB / km, or even less than 2 dB / km in the wavelength range 1000 to 1200 nm. The optical fiber according to the invention is usable in low bandwidth applications, such as single channel applications, as well as in high bandwidth applications, such as wavelength division multiplexing applications (these examples not being limiting), depending on the amount of complementary doping elements contained in the optical fiber, while remaining sufficiently insensitive to irradiation.

In one embodiment, the nanoparticle matrix does not comprise complementary doping elements. For example, the nanoparticle matrix is silica. The rare earth concentration is between 150 and 250 mass ppm.

R: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11:11 - 16/33 The optical fiber has an induced attenuation under irradiation of between 0.005 dB / m / kGy and 0.05 dB / m / kGy. In one embodiment, the nanoparticle matrix comprises complementary doping elements that enhance the dissolution of rare earths in high concentrations and the gain properties of the optical fiber, and maintain a physical barrier between the rare earths. The complementary doping elements make it possible to obtain a large bandwidth, without penalizing the resistance to irradiation. In a non-limiting example of this embodiment, the nanoparticle matrix is an oxide such as alumina, which allows a good distribution of the rare earths in the nanoparticle, and makes it possible to widen the spectral window with amplification gain. for wavelength division multiplexing applications. In this example, the core 11 comprises an amount of rare earths of between 200 and 400 mass ppm and a quantity of complementary doping elements of between 2.5 and 3.5% by weight. This allows a compromise between an erbium-related attenuation, proportional to the amount of erbium present in the fiber, and the bandwidth. Thus, in the case of doping with erbium, the optical fiber exhibits erbium-related attenuation per unit length of between 3 and 6 dB.rri-1 at the wavelength 1530 nm, and bandwidth between 25 and 32 nm. The optical cladding comprising the holes 10 carries a part of the fundamental optical mode and can disturb the signal transmitted by the central core 11. The contribution of the optical cladding in the transmitted signal decreases rapidly with the radius of the optical fiber. In practice, the portion comprising the first ring of holes 10 around the central core 11 contributes mainly to the transport of the optical signal. In the example shown in FIG. 8, the zone likely to disturb the transmitted signal is an annular zone between the 0.5At radii. and 1.5At. In one embodiment, the optical cladding has a chlorine concentration of less than 500 mass ppm, preferably less than 100 mass ppm.

The optical cladding is furthermore devoid of other chemical elements detectable by conventional chemical analysis means. The plasma torch or ICP (Inductively Coupled Plasma) is an example of a chemical analysis. For example, the optical cladding is devoid of other chemical elements having a

R: 131900A31905 AOB \ 31905--101125-text deposit.doc - 25/11/10 - 11:11 - 17/33 concentration greater than one part per billion mass. In other words, in the optical cladding, the concentration of chemical elements other than chlorine is less than one part per billion mass. Thus, the optical cladding has a limited quantity of radiation-sensitive chemical elements, which would attenuate the signal transmitted by the central core 11. Thus, the optical cladding has attenuation of the signal minimized under irradiation. The optical cladding is devoid of chemical elements inducing losses under irradiations greater than 0.05 dB / m / Gy. In one embodiment, the zone 12 doped with rare earths of diameter (ledopage does not cover the whole of the central core 11. In this embodiment, only one of the overlap between the central core 11 and the beam pump optic, and the overlap between the central core 11 and the signal optical beam is improved, for example, the overlap is between 0.6 and 0.7 In one embodiment, the central core 11 and the holes 10 show only rotation symmetry of order n modulo n around the center of the optical fiber (denoted n [n]), where n is an integer, such rotation symmetry of order n [n] allows a conservation of the polarization of the signal transmitted in the optical fiber Typically, in an optical fiber where the index difference between the central core and the optical cladding is obtained with doping elements, the polarization of the transmitted signal is maintained by inserting into the optical sheath, on both sides of the central core, bars of silica doped with boron. In this embodiment of the optical fiber according to the invention, the optical cladding makes it possible to obtain a polarization-maintaining optical fiber without resorting to additional dopants such as boron. FIGS. 2 to 4 show examples of optical fibers in which the holes 10 and the central core 11 only have symmetries of order n [n]. The example of optical fiber in FIG. 8 has a symmetry of order n / 3. The example of optical fiber in FIG. 8 therefore does not only have a symmetry of order n [n] and is not a polarization-maintaining optical fiber. The advantages of the optical fiber according to the invention will be better understood with the aid of the examples of amplifying optical fibers I, II, III and IV presented in Tables 1 and 2.

R: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11:11 - 18/33 Examples I and III are optical fibers of the prior art in which the index difference between the central core and the optical cladding is obtained with doping elements, such as germanium and / or fluorine. The doping of the central core with rare earths is obtained by impregnation of a solution containing rare earths without the use of nanoparticles. Examples of optical fibers II and IV are optical fibers according to the invention. The confinement of the signal in the central core 11 is obtained with the holes 10 of the optical cladding. In particular, in the optical fibers II and IV the holes 10 are organized in a triangular array known per se. The optical fibers II and IV are further doped with erbium via nanoparticles. Table 1 shows the examples of optical fibers I, II that can be used in a low-bandwidth application, such as a single-channel application. Table 1 Fiber I Fiber II 960 nm <960 nm 2r1 2.5 μm No Afro '6.0 μm On 30 .10-3 c ~ hole 2.4 μm 1) hole / Atrou 0.4 [Er] 1.6x 1024 ions / m3 [Er] 1.6x 1024 ions / m3 2r1 6.0 μm OEl530 [Er] 3 dB.m 1 OE1530 [Er] 4 dB.m 1 On doping 0 [Al] - 0.3% by weight [Al] 0 [Ge] -30% by weight [Ge] 0 x980 nm 0.82 x980 nm 0.68 x1550 nm 0.53 x1550 nm 0.66 Length of the fiber 30 m Length of the fiber 23 m optical amplifying optics Amplifier For each optical fiber, Table 1 shows the cut-off wavelength; Erbium concentration [Er] in the central core; the diameter 2r1 of the central heart; the concentration of aluminum [Al]; the concentration of germanium [Ge]; the recoveries 198o nm and F1550 nm between the doped section and respectively the pump beam of wavelength 980 nm, and the wavelength signal beam 1550 nm; and the length of the amplifying optical fiber. Table 1 also shows the attenuation per meter OE1530 [Er] due to the incorporation of the erbium, measured at 1530 nm wavelength. This attenuation is different from the attenuation induced under irradiation. The attenuation per unit length OE1530 [Er] depends on the erbium concentration and the profile of the optical fiber. For optical fiber I, Table 1 also shows the difference in index An between the central core and the optical cladding. For the optical fiber II, Table 1 furthermore has the pitch At. Between the holes 10 of the optical cladding, the diameter (Dtrou holes 10, the ratio (Dtrou / Atrou between the diameter (Dtrou and Atrou pitch), and the doping index difference An doping caused by the complementary dopants inserted in the central core In the optical fiber I, the confinement of the signal in the central core 11 is obtained by doping the central core 11 with germanium which contributes to the difference of index An of the heart.

In optical fiber II, the nanoparticles consist of a silica (SiO 2) and erbium matrix. Doping with nanoparticles avoids the use of aluminum to avoid rare earth aggregates. The doped zone 12 therefore does not include doping elements which influence the index difference. Thus the difference of index An doping is equal to zero. The pitch Anu between the holes 10 of the sheath is 6 μm. This value makes it possible to obtain a compromise between the effective surface of the optical fiber, the overlap between the doped zone 12 and the signal and pump beams, and the resistance to bending losses. The ratio (Dtrou / Atrou between the diameter (Dtrou and pitch At.) Is 0.4.This value ensures stability of the monomode behavior of the optical fiber, and a cut-off wavelength Xc less than the length of the optical fiber. 960 nm pump wave The diameter 2r1 of the central core 11 is optimized to a maximum value to maintain the ratio (Dtrou / Atrou in the passage from the preform to the optical fiber during the manufacture of the optical fiber. I and II have the same concentration of erbium.

However, by its characteristics, the optical fiber II has fewer radiation-sensitive dopants than the optical fiber I. Indeed, the rare earth doping by nanoparticles makes it possible to dispense with the use of aluminum; the holes R: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11:11 - 20/33 10 in the optical cladding make it possible to dispense with the use of germanium. Thus, optical fiber II does not have dopants sensitive to irradiations other than erbium. In addition, the holes 10 make it possible to improve the overlap between the signal and the zone 12 doped with erbium, and thus to reduce the length of optical fiber. Thus, the optical fiber II according to the invention has an optical fiber length exposed to irradiations lower than that of the optical fiber I. The optical fiber II according to the invention therefore has a decrease in the sensitivity to irradiation with respect to the fiber optical I of the prior art. The attenuation per meter a153O [Er] due to the incorporation of erbium, measured at the wavelength 1530 nm, of the optical fiber II is greater than that of the optical fiber I because of the improvement of the overlap between the doped zone and the optical signal at the wavelength considered. Table 2 presents examples of optical fibers III, IV that can be used in a high bandwidth application, such as a wavelength division multiplexing application. The aluminum concentration is higher than in optical fiber examples I and II to increase the bandwidth. Table 2 Fiber III IV fiber 960 nm <960 nm 2r1 3.2 μm No Afro '6.0 μm On 19 .10-3 c ~ hole 2.4 μm 1) hole Atrou 0.4 [Er] 2.0X1024 ions / m3 [Er] 2.0x1024 ions / m3 2r1 6.0 μm OEl530 [Er] 4 dB.rri-1 OE153O [Er] 6.1 dB.rri-1 On doping 10 X 10-3 [Al] -7 % by weight [Al] - 3% by weight [F] -1.0% by weight [F] 0 [Ge] - 0.5% by weight [Ge] 0 x 980 nm 0.83 x 980 nm 0.92 x 1550 nm 0.55 x1550 nm 0.84 Length of the fiber 23 m Length of the fiber 15 m optical optics R: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11:11 - 21/33 Table 2 shows the same parameters as Table 1. For each optical fiber, Table 2 also shows the fluorine concentration [F] of the optical fiber. In the optical fiber III, the index difference between the central core and the optical cladding is obtained essentially with the inserted aluminum to avoid the aggregation of the rare earths. The index difference is furthermore obtained with the complementary codopage of the central core with germanium and doping of the cladding with fluorine. In the optical fiber IV, the nanoparticles consist of an alumina matrix comprising erbium. Alumina induces a difference in the index of the ondopage. The characteristics of the pitch At. Between the holes, of the ratio (D> rou / Afro 'between the diameter (I)> rot, and the pitch At., And of the diameter (OD) are identical to those of the optical fiber II. optical fibers III and IV have the same concentration of erbium.

However, by its characteristics, the optical fiber IV has fewer radiation-sensitive dopants than the optical fiber III. The rare earth doping by nanoparticles makes it possible to reduce the quantity of alumina necessary to obtain a broad band. The holes 10 of the optical cladding also make it possible to eliminate the fluorine and germanium usually required to obtain the guiding properties. Thus, the optical fiber IV has a quantity of dopants sensitive to irradiation lower than that of the optical fiber III. The holes 10 of the optical cladding also make it possible to reduce the length of optical fiber. Thus, the optical fiber IV according to the invention has an optical fiber length exposed to irradiations lower than that of the optical fiber III. The optical fiber IV according to the invention thus has a decrease in sensitivity to irradiation compared to the optical fiber III of the prior art. The attenuation per meter a1530 [Er] due to the incorporation of erbium, measured at the wavelength 1530 nm, of the optical fiber IV is greater than that of the optical fiber III because of the improvement of the overlap between the doped zone and the optical signal at the wavelength considered. The invention also relates to an optical amplifier comprising at least a portion of the optical fiber according to the invention, and using a pump power 8: 131900/31905 AOB / 31905--101125-texte dépôt.doc - 25/11/10 - 11:11 - 22/33 between 150 mW and 1.5 W. In one embodiment, the amplifier has a bandwidth of 28 to 32 nm in the C band (1530 to 1560 nm). The invention also relates to a method for manufacturing a primary preform of an optical fiber such as that described above. The method will be better understood with reference to Figs. 5 to 7. As shown in Fig. 5, the method comprises a step of forming a portion 200 of the preform comprising nanoparticles doped with rare earths. This part 200 of the preform contributes to forming the central core 11 of the optical fiber after drawing the preform. For example, the rare earth concentration in this part 200 of the preform is between 200 and 1000 mass ppm and the concentration of the matrix of nanoparticles is between 0.5 and 5% by weight, or even advantageously between 1.5 and 4% by weight. Part 200 of the preform has a chlorine concentration of less than 1000 mass ppm. Part 200 is further free of other detectable chemical elements by conventional chemical analysis means. The plasma torch or ICP (Inductively Coupled Plasma) is an example of a chemical analysis. For example, part 200 is devoid of other chemical elements having a concentration greater than one part per billion mass. By minimizing the amount of chlorine and other chemical elements in the portion 200, the irradiation resistance of the optical fiber obtained after drawing is improved. The method then comprises a step 36 for manufacturing capillaries 100. These capillaries 100 define the holes 10 of the optical fiber obtained after stretching the preform. A step 38 is then carried out, in which the capillaries 100 are arranged in a bundle of capillaries, with the portion 200 of the preform at the center of the bundle of capillaries. Thus, the preform comprises elements which, after drawing, respectively constitute the holes 10 and the central core 11 of the optical fiber. The method makes it possible to obtain a preform which, after drawing, will be an optical fiber having a central core 11 comprising nanoparticles doped with rare earths and a perforated sheath. The method according to the invention makes it possible to obtain a preform of an optical fiber resistant to irradiation and having optimum amplification performance.

R: 131900A31905 AOB \ 31905--101125-texte dépôt.doc - 25/11/10 - 11:11 - 23/33 The preform then undergoes the conventional steps of a multimode optical fiber. For example, the assembly consisting of the portion 200 and the capillaries 100 is completed by bars 300 and a sheath 400 forming the outer sheath after stretching. In a first embodiment, the capillaries 100 have a chlorine concentration of less than 500 mass ppm, preferably less than 100 mass ppm. The capillaries 100 are furthermore devoid of other detectable chemical elements by conventional chemical analysis means such as the plasma torch. For example, the capillaries 100 are devoid of other chemical elements having a concentration greater than one part per billion mass.

In other words, in the capillaries 100, the concentration of chemical elements other than chlorine is less than one part per billion mass. Thus, the capillaries 100 have a limited quantity of chemical elements sensitive to irradiation, which would attenuate the signal transmitted by the central core 11 of the optical fiber obtained after stretching. For example, the capillaries 100 are pure silica.

In a second embodiment shown in FIG. 6, step 35 comprises a step 30 of performing a porous deposit on the inner surface of a tube 210. The porous deposit, which follows this inner surface, is therefore of tubular form. . For example, the porous deposit is pure silica. For example, the deposition is obtained using a modified chemical vapor deposition technique MCVD ("Modified Chemical Vapor Deposition"). In this embodiment, step 35 then comprises a step 31 comprising impregnating the porous deposit with a suspension of nanoparticles. The nanoparticles are doped with rare earths. In this embodiment, step 35 then comprises a step 32 of vitrifying the porous deposit at high temperature, for example at a temperature of 2000 ° C. In this embodiment, step 35 then comprises a step 33 of necking of the assembly consisting of the tube 210 and the porous deposit. Initially the porous deposit is of tubular form. In step 33, the porous deposit is closed. Thus, at the end of this step 33, the porous deposit no longer has a tubular shape, but takes the form of a solid cylinder. In this same embodiment, step 35 then comprises a step 34 of extracting the porous deposit. In other words, the tube 210 is removed. The tube

R: 131900A31905 AOB \ 31905--1011254exposed deposit.doc - 25/11/10 - 11:11 - 24/33 210 is not preserved in the optical fiber obtained after drawing the preform. Thus, the level of impurities in the tube 210 is not restrictive on the optical characteristics of the optical fiber after drawing. By way of non-limiting examples, the tube 210 may be removed by evaporation, machining or etching, or by a combination of these three techniques. The porous deposit extracted from the tube 210 thus constitutes the portion 200 of the preform comprising nanoparticles doped with rare earths and which contributes to forming the central core 11 of the optical fiber after drawing. In a third embodiment, step 35 comprises a step of forming a bar comprising nanoparticles doped with rare earths. The bar is for example in the form of a solid cylinder. For example, the bar is based on pure silica. For example, the bar is made via a sol-gel process known per se. In this embodiment, step 35 then comprises a vitrification step at a temperature of 100 ° C. and then a step of shrinking of the bar. In this embodiment, the bar thus constitutes the portion 200 of the preform comprising nanoparticles doped with rare earths and which contributes to forming the central core 11 of the optical fiber after drawing. In a fourth embodiment, the step 36 of making the capillaries 100 comprises the steps described hereinafter with reference to FIG. 7. The step 36 of making the capillaries 100 comprises a step of making a deposit on the internal surface of a tube 110. The deposit is for example made using a physicochemical vapor deposition technique PCVD ("Plasma-activated chemical vapor deposition" in English).

The step 36 of manufacturing the capillaries 100 then comprises a step 22 of extracting the tubular deposit, that is to say, to remove the tube 110 to keep only the deposit. By way of nonlimiting example, the tube 110 is removed by machining, etching or a combination of these two methods. The capillaries 100 (step 24) are then drawn from the tubular deposit. The tube 110 is not preserved in the capillaries 100 obtained. Thus, the impurity level of the tube 110 is not restrictive on the optical characteristics of the optical fiber after drawing.

In a process according to the first and the fourth embodiment, the deposition on the inner surface of the tube 110 is in a process according to the first and the fourth embodiment. has a chlorine concentration of less than 500 mass ppm, preferably less than 100 mass ppm. The deposit is further free of other detectable chemical elements by conventional chemical analysis means such as the plasma torch. For example, the deposit is devoid of other chemical elements having a concentration greater than one part per billion mass. By minimizing the amount of chlorine and other chemical elements in the deposit, the irradiation resistance of the optical fiber obtained after drawing is improved.

In a fifth embodiment, the step 36 of making the capillaries 100 comprises the step of depositing on the inner surface of a tube 110, as described above. In this embodiment, step 36 then comprises pulling the capillaries 100 (step 24) from the assembly consisting of the tube 110 and the tubular deposit. In this embodiment, the tube 110 is preserved in the capillaries 100 obtained. Thus, the manufacturing process of the capillaries 100 is simplified. In a method according to the first and fifth embodiments, the tube 110 and the deposition on the inner surface of the tube 110 have a chlorine concentration of less than 500 mass ppm, preferably less than 100 mass ppm.

The tube 110 and the deposit are furthermore devoid of other detectable chemical elements by conventional chemical analysis means such as the plasma torch. For example, the tube 110 and the deposit are devoid of other chemical elements having a concentration greater than one part per billion mass. By minimizing the amount of chlorine and other chemical elements in the tube 110 and the deposition, the irradiation resistance of the optical fiber obtained after drawing is improved. The nanoparticles are for example produced by chemical or physical synthesis. Advantageously, the nanoparticles are produced by a chemical synthesis, which promotes the formation of thermodynamically stable stoichiometric structures. A standard chemical method can be used to chemically synthesize the nanoparticles in a controlled pH aqueous solution, by coprecipitation of alumina salt precursors (in the case where the

R: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11:11 - 26/33 nanoparticle is made of alumina) and rare earth salts. For example, the precursors of alumina are inorganic salts such as nitrate or chloride, and the precursors of erbium, ytterbium or thulium are organic salts such as acetyl acetonate or acetate. For example, the atomic ratio of the precursors of alumina salts and of rare earth salts is between 10 and 500, or even between 50 and 350, in order to obtain a concentration by weight of the rare earths in each nanoparticle which is between 0.5 and 3% by weight, or even between 0.75 and 1.5% by weight depending on the intended applications and the choice of the rare earth. In one example, the atomic ratio of the precursors of alumina salts and rare earth salts is even between 50 and 200. In a particular embodiment, the matrix of the nanoparticles is alumina, the rare earth is erbium, the atomic ratio between alumina and erbium being between 10 and 500, even between 50 and 350 or even between 50 and 200. The nanoparticles are then washed and dispersed in an aqueous or alcoholic solution with a concentration nanoparticles between 1016 and 1018 per cm3 depending on the size of the nanoparticles. For example, the nanoparticles have a substantially spherical shape and a diameter of between 5 and 25 nm. A tolerance of 20% is acceptable on the characteristics of nanoparticles. For example, the nanoparticles are dispersed in the aqueous or alcoholic solution with a nanoparticle concentration greater than or equal to 1017 per cm3, for nanoparticles 5 nm in diameter, and greater than or equal to 1016 per cm3, for 10 nm nanoparticles. of diameter. In order for the matrix of each nanoparticle to be preserved in the final optical fiber and to constitute a physical barrier between the rare earths, it is important that it can withstand the conditions (temperature and constraints) of manufacturing the optical fiber. In a method according to the second embodiment, the method comprises, for example, a step of thermal densification of the nanoparticles, after their incorporation by impregnation into the porous deposit 210 and before the vitrification of the deposit. The deposit 210 underwent a heat treatment at a temperature above 1000 ° C for at least 1 hour, to strengthen the structure of the nanoparticles. R: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11:11 - 27/33

Claims (23)

REVENDICATIONS1. An optical fiber comprising, from the center to the periphery: a central core (11) adapted to transmit and amplify an optical signal, the central core (11) consisting of a core matrix comprising nanoparticles, the nanoparticles being formed of a nanoparticle matrix comprising doping elements of the rare earth group; an optical cladding surrounding the central core (11) adapted to confine the optical signal transmitted by the central core (11), the optical cladding having a plurality of holes (10) which extend along the length of the optical fiber; holes (10) being separated from each other by one step (Afro '); - an outer sheath. An optical fiber according to claim 1, wherein the optical cladding has a chlorine concentration of less than 500 mass ppm and is devoid of other chemical elements having a concentration greater than one part per billion mass. An optical fiber according to claim 1, wherein the optical cladding has a chlorine concentration of less than 100 mass ppm and is devoid of other chemical elements having a concentration greater than one part per billion mass. 4. Optical fiber according to one of the preceding claims, wherein the optical cladding is pure silica. Optical fiber according to one of the preceding claims, in which the holes (10) and the central core (11) have only n modulo n rotation symmetries around the center of the optical fiber, where n is an integer. Optical fiber according to one of the preceding claims, wherein the pitch (AirOU is between 2 μm and 10 μm.) R: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11: 11 - 28/33 Optical fiber according to one of the preceding claims, wherein the holes (10) have a substantially circular cross-section, and each hole (10) has a diameter ((Dtro ') such that the ratio between the diameter ((Dtro ') and the pitch (Afro') is between 0.3 and 0.9. 8. Optical fiber according to one of the preceding claims, wherein the core matrix (11) is pure silica. Optical fiber according to one of the preceding claims, in which the central core (11) has a rare earth element dopant concentration of between 200 and 1000 mass ppm, and a nanoparticle matrix concentration of between 0 and 5 and 5% by weight. 10. Optical fiber according to one of the preceding claims, wherein the nanoparticles have an atomic ratio between the nanoparticle matrix and the doping elements of the rare earth group between 10 and 500, preferably between 50 and 350. 11. Optical fiber according to one of the preceding claims, wherein the nanoparticle matrix is a material selected from alumina, silica, or a combination thereof. Optical fiber according to one of the preceding claims, wherein the rare earth element doping elements are selected from among erbium, ytterbium, thulium or a combination thereof. 13. Optical amplifier comprising at least one portion of optical fiber according to one of the preceding claims and using a pump power of between 150 mW and 1.5 W. 14. Optical amplifier according to the preceding claim, having a bandwidth of between 25 and 32 nm for the C band (from 1530 to 1560 nm). Optical fiber optical laser comprising at least one optical fiber portion according to one of claims 1 to 12. R: 131900A31905 AOB \ 31905--101125-texte dépôt.doc - 25/11/10 - 11:11 - 29/33 16. A method of manufacturing a primary optical fiber preform, comprising the steps of: forming (35) a portion (200) of the preform comprising nanoparticles doped with elements of the rare earth group, said part (200) ) of the preform contributing to form the central core (11) of the optical fiber; - manufacturing (36) a plurality of capillaries (100) - arranging (38) the capillaries (100) into a bundle of capillaries comprising said portion (200) of the preform at its center. 17. The method of claim 16, wherein said portion (200) of the preform contributing to form the central core (11) of the optical fiber is made of a pure silica material. 18. The method of claim 16 or 17, wherein the capillaries (100) have a chlorine concentration of less than 500 mass ppm and are free of other chemical elements having a concentration greater than one part per billion mass. 19. The method of claim 16 or 17, wherein the capillaries (100) have a chlorine concentration of less than 100 mass ppm and are free of other chemical elements having a concentration greater than one part per billion mass. 20. Method according to one of claims 16 to 19, wherein the capillaries (100) are pure silica. 21. The method according to one of claims 16 to 20, wherein the forming step (35) comprises the steps of: - performing (30) a porous deposit on the inner surface of a tube (210), said deposit 25 having a tubular shape; impregnating (31) said porous deposit with a suspension of nanoparticles doped with elements of the rare earth group; - Vitrify (32) said porous deposit, - shrink (33) the assembly consisting of the tube (210) and the porous deposit; Extracting (34) said porous deposit, said deposit constituting said portion (200) of the preform contributing to forming the central core (11) of the optical fiber. R: 131900A31905 AOB \ 31905--101125-text filing.doc - 25/11/10 - 11:11 - 30/33 22. The method of claim 21, wherein the step (30) of deposition is carried out through a modified chemical vapor deposition technique MCVD ("Modified Chemical Vapor Deposition" in English). 23. The method according to one of claims 16 to 20, wherein the forming step (35) comprises the steps of: - forming a bar comprising nanoparticles doped with elements of the rare earth group; - vitrify the bar; - Tighten the bar, said bar constituting said portion (200) of the preform 10 helping to form the central core (11) of the optical fiber. 27. The method of claim 23, wherein the bar is made via a sol-gel process. 28. The method according to one of claims 16 to 24, wherein the step (36) of manufacturing the plurality of capillaries (100) comprises the steps of: - performing (20) a deposit on the inner surface of a tube (110), said deposit having a tubular shape; extracting (22) the tubular deposit; pulling (24) capillaries (100) from the tubular deposit. 26. The method according to one of claims 16 to 24, wherein the step (36) of making the plurality of capillaries (100) comprises steps of: - performing (20) a deposit on the inner surface of a tube (110), said deposit having a tubular shape; and pulling (24) capillaries (100) from the tube (110) comprising the deposit on its inner surface. 27. The method according to claim 25 or 26, wherein the depositing step (20) is performed by means of a PCVD (Plasma Chemical Vapor Deposition) vapor-phase physicochemical deposition technique. in English). 28. The method according to one of claims 16 to 27, wherein the concentration of nanoparticles in the portion (200) of the preform is between 1016 and 1018 / cm3. R: 131900A31905 AOB \ 31905--101125-text deposit.doc - 25/11/10 - 11:11 - 31/3329. Process according to one of Claims 16 to 28, in which the nanoparticles have a substantially spherical shape and a diameter of between 5 and 25 nm. 8: 131900/31905 AOB / 31905--101125-text filing.doc - 25/11/10 - 11:11 - 32/33